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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>
    // 1. 声明一个年龄变量
    let age
    // 2. 赋值
    age = 18
    console.log(age);
  </script>
</body>
</html>